<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <input type="text" class="text">
  <button class="btn">按钮</button>
  <script>
    var oInp = document.querySelector(".text");
    var oBtn = document.querySelector(".btn")
    // 设置value的值
    // oInp.value = "123456";

    oBtn.onclick = function(){
      // 返回表单控件的value值
      var txt = oInp.value;
      console.log(txt);
    }
  </script>
</body>
</html>